Radiation Heat Transfer

Heat transfer due to emission of electromagnetic waves is known as thermal radiation

Heat transfer through radiation takes place in the form of electromagnetic waves, mainly in the infrared region. The radiation emitted by a body is the consequence of thermal agitation of its composing molecules. The radiation of heat can be described by the reference to the so-called 'black' body.

The Black Body

A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material. The emission spectrum of such a black body was first fully described by Max Planck.

The energy radiated by a blackbody is proportional to the fourth power of the absolute temperature and is given by Stefan-Boltzmann Law:

q = σ T⁴ A (1)

where

q = heat transfer (W)

σ = 5.6703 10^-8 (W/m².K⁴) - The Stefan-Boltzmann Constant

T = absolute temperature Kelvin (K)

A = area of the emitting body(m²)

• A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.

Emissivity Coefficient

For objects other than ideal blackbodies the Stefan-Boltzmann Law can be expressed as:

q = ε σ T⁴ A (1)

where

ε = emissivity of the object (= 1 for a black body)

The emissivity lies in the range 0 < ε < 1 and depends on the type of material and the temperature of the surface. The emissivity of some common materials are:

• oxidized Iron at 390 ^oF (199 ^oC) - ε = 0.64
• polished Copper at 100 ^oF (38 ^oC) - ε = 0.03
• emissivity coefficients for some common materials

The Net Radiation Loss Rate

If a hot object is radiating energy to its cooler surroundings, the net radiation loss rate can be expressed as:

q = ε σ (T_h⁴ - T_c⁴) A (3)

where

T_h = hot body absolute temperature

T_c = cold surroundings absolute temperature

• Radiation Constants for some common Building Materials

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